This invention relates to the manufacture of printed circuit boards and more paticularly to a new and novel process for manufacturing these boards. Much has been written about printed circuit board manufacture and many patents have been granted in this large and rapidly growing industry. These boards have become very important commercially, since they are used in almost every area of the electronics industry and form an important part of almost every electronic device that is made. Printed circuitry has become a most economical and viable technique for electrically connecting the various components that make up any device that utilizes electronics.
In order to make a printed circuit board, the starting material is usually a board consisting of fiberglass cloth and epoxy resin. In this form, the board is a dielectric, thus preventing short circuits from taking place between the various electronic components that are attached to the board. In place of epoxy resin, it is possible to use a phenolic resin or other suitable resins. In place of the fiberglass cloth, it is possible to use compressed paper or other suitable materials. The basic board is therefore a flat, rigid or slightly flexible, dielectric material that is suitable to be fabricated into the final printed circuit. The most common form of starting material for making these boards is a plastic sheet or board which also contains a thin layer of copper laminated to both sides of the board with suitable adhesion. This combination is commonly called copper laminate.
Holes. In most casts, holes are provided through these boards (usually by drilling) to accommodate the leads of the various electronic components that will be attached thereto. The holes are drilled using high speed drilling machines and the locations of the holes are specified in the drawings or blueprints for the boards.
Metallizing. In order to make an electrical connection from one side of the copper laminate through the holes to the other side, the plastic wall of the hole must now be made conductive. This is accomplished by a chemical process commonly known in the industry as metallization, and consists of a relatively complicated series of chemical tanks and rinses and an activating step to apply a thin copper layer to the hole walls.
Imaging. The image is a circuit design that has been applied to the surface of the drilled board in accordance with the requirement of the specifications or blueprints. The image is an organic coating applied either as a dry film (photo resist) or a screened ink. The organic coating forms the circuit design pattern by protecting only the surface onto which it is applied in order to prevent chemical attack on these surfaces during processing.
Electroplating. Since the copper layer formed by the metallizing process is generally too thin to form a suitable electrical bridge between the two copper layers of the board, copper electroplating is used to deposit a heavy layer of copper in the holes in order to form a suitable copper cross section for carrying current. Copper plating is generally followed by tin-lead or tin plating in order to supply both solderability and a surface that chemically resists the etching solution that is used in the etching step that follows.
Etching. When the organic coating that forms the image is stripped off the thin layer of bare copper is exposed. This bare copper is now completely etched away down to the plastic leaving a board that now has an electrical circuit firmly attached to the plastic surface and drilled holes that are electrically connected and suitable for accepting the electronic components that will be mounted onto it.